1. Field of the Invention.
This invention is in the field of circuits that dynamically enhance a two-channel stereophonic sound signal such as the stereophonic sound images produced by loudspeakers or headphones. These circuits enhance the sound by generally making it wider and more spacious.
2. Prior Art.
Often stereophonic image widening circuits are designed to ensure monophonic compatibility when the stereophonic signal is broadcast. That is, the sum of the left and right stereophonic channels (often termed "L+R") is not affected by the processing, and the circuit does not significantly affect the peak modulation level of a standard FM stereophonic broadcasting system. This ensures that full modulation is retained for listeners with monophonic receivers, and that the modulation accurately represents the sum of the original left and right channels.
In general, the prior circuits can be divided into two classes: (1) those which process the signal in such a way that the L+R signal is affected by the processing, and (2) those which process the signal such that L+R is unaffected. In the latter case, only the stereophonic difference signal (L-R) is affected. (For convenience, these latter circuits are referred to as having "unaffected L+R".) The present invention is in this latter category.
The simplest form of circuit with unaffected L+R(Prior Art Circuit 1) matrixes the stereophonic signal into sum-and-difference (L+R/L-R) form, amplifies the L-R signal, and then re-matrixes the amplified L-R and the original L+R into an enhanced discrete left and right (L/R) form.
Mathematically, if the gain applied to the L-R signal is k, then enhanced L=0.5(L+R)+0.5k(L-R)=0.5k(k+1)-0.5R(k-1), and enhanced R=0.5(L+R)-0.5k(L-R)=0.5R(k+1)-0.5L(k-1) k=1 for no enhancement
Thus, this enhancement process has two effects: (1) it increases the level of the desired channel by a gain factor of 0.5(k+1) and (2) it introduces out-of-polarity crosstalk from the undesired channel at a gain of 0.5(k-1). It is the introduction of out-of-polarity crosstalk symmetrically into the opposite channel that causes the perceived widening on the stereophonic image.
While the relatively simple processing of this Prior Art Circuit 1 does indeed increase the perceived width of the signal, it suffers from two problems that make it unsuitable for broadcast use:
First, if the level of the L-R signal prior to processing is a significant fraction of the level of the L+R signal, then after processing, the level of the L-R signal can substantially exceed the level of the L+R signal. This is not a satisfactory situation in FM stereo broadcasting by the world-standard "pilot-tone" method. The difficulty arises because the peak modulation of the stereophonic composite baseband signal is proportional to the peak level of the higher of the left or right channels. Examination of the mathematical expressions for the enhanced L and enhanced R signals above shows that the levels of these signals increase with increasing k. The factor by which the peak level increases can be as large as 0.5(1+k). Meanwhile, because the Prior Art Circuit 1 operates only the L-R signal, the level of the L+R is not affected. If, after processing, the level of the L-R signal is significant by comparison to the level of the L+R signal, the peak levels of the enhanced left and right signals (and thus the peak level of the stereophonic composite baseband signal) are increased. This occurs without any corresponding increase in the level of the L+R signal. Because the peak level of the composite baseband signal must be constrained so that it does not exceed a certain level (determined usually by government broadcasting authorities so that interference to adjacent channels is prevented), this means that the potential overall L+R modulation is reduced by addition of excessive L-R level. This reduces the loudness and signal-to-noise ratio available at monophonic receivers, which reproduce only the L+R signal. This situation is unacceptable to broadcasters. PA1 As a matter of good engineering practice, the L-R to L+R ratio should be constrained to unity if the signal is to be considered "broadcastable". (This corresponds to a signal in only one stereophonic channel.) PA1 A second problem with the Prior Art Circuit 1 involves reverberation and other ambient sounds which often have most of their energy in the L-R channel. Fixed-gain amplification of the L-R channel has the effect of exaggerating reverberation and ambience, sometimes to the point of making the signal sound excessively reverberant or "muddy". In addition, the L-R channel is more likely than the L+R channel to suffer from noise and distortion, particularly if its source is a phonograph record. Fixed-gain amplification of the L-R channel can unpleasantly exaggerate such noise and distortion as well.
Another prior art circuit (Prior Art Circuit 2) solves the problems discussed above for Prior Art Circuit 1 by placing a voltage-variable-gain amplifier (VCA) in series with the L-R signal path. The gain of the VCA is reduced whenever a control means that monitors the ratio of L-R to L+R determines that this ratio is too high (usually meaning greater than unity).
Additional control means are used to reduce the gain of the VCA when the input signal is determined to be monophonic (usually, by measuring the ratio of L-R to L+R and assuming that the signal is monophonic if this ratio is sufficiently small). In this case, any small amount of residual L-R is assumed to be noise or an artifact of slight imbalance between the gains or phases of the left and right channels. It is clearly undesirable to amplify such noise or to exaggerate any channel imbalance errors.
In a variation of Prior Art Circuit 2, the L-R signal only is extracted from the original left and right signals by means of a matrix. (The L+R signal is not derived.) The L-R signal is applied to a VCA, the output of which ("incremental enhanced L-R") is added to the original left channel and subtracted from the original right channel to form enhanced left and right channels. It can be shown that this is mathematically identical to the process of extracting the L+R and L-R signals, amplifying the L-R signal, and re-matrixing to form the enhanced left and right signals:
enhanced L=L+c(L-R)=L(1+c)-cR, and
enhanced R=R-c(1-R)=R(1+c)-cL
where c is the gain of the VCA (c=o for no enhancement).
For equivalent ratios of original-to-crosstalk signals Prior Art Circuit 1 and this variation to Prior Art Circuit 2, c=(1-k)/2k where k is as defined above for Prior Art Circuit 1.
While, in this variation of the Prior Art Circuit 2, the level of the enhanced L-R signal can never be less than the level of the original L-R, this is satisfactory because the object of the circuit is to enhance the L-R signal; the L-R signal is never reduced in level. The advantage of this circuit is that only the incremental enhanced part of the L-R signal is passed through the VCA (not the entire L-R signal), and thus any noise and/or distortion contributed by the VCA are applied only to the incremental enhanced portion of the L-R signal. A further advantage is that when the gain of the VCA is zero, perfect separation is achieved at the circuit's output: there is no possibility of loss of separation due to errors in matrix and dematrix circuitry.
Still another variation of Prior Art Circuit 2 firstr applies the L-R signal to a delay line. The original L-R signal and its delayed version are comboned in userdetermined proportion, and then applied to a VCA that operates in the same manner as in the preferred circuit described immediately above. Thus the incremental enhanced L-R may consist not only of a signal directly proportional to the original L-R, but also to the original L-R as applied to a time delay means.
Neither of the variations to Prior Art Circuit 2 sloves the second problem discussed above for the Prior Art Circuit 1 (excessive exaggeration of reverberation and ambience) because the gain of the L-R channel is always greater than unity.